They should just kill the format. Everything should just be drag to install, drag to trash to remove.
It was actually a really beautiful synergy—you install applications by copying them to a folder, and launch them from that folder. Same way you'd acquire and open files. Lovely.
Then Apple ruined it in Lion with Launchpad. Their app install flow for anything outside of the app store doesn't make any sense.
I wish it were that easy, most apps leave files in other places on your computer like ~/Library that will never get cleaned up if you just move the app to trash.
The DMGs are a clever way to (A) make sure the app gets to the proper location while simultaneously (B) teaching the user about what's actually happening on their computer. As I said in a sibling comment, this all made much more sense when users also launched apps from the Applications folder directly.
It might be nice if macOS had some sort of automatic cleanup routine when an app is trashed, but that would either require showing the user an extra dialog (a la AppCleaner's) or introducing an opaque system which could potentially lead to data loss.
Some applications offer to move themselves to the /Applications folder when started the first time outside /Applications or ~/Applications. Though in general, it would be better if Apple made it more attractive to publish in the App Store, since it brings other advantages (e.g. mandatory sandboxing).
But I think the primary argumentation in favor of what macOS does now on drag-to-trash is that the users preferences are preserved, for when they install an application again.
Also, personally, I sometimes purposefully put apps in places other than /Applications—for example, I like to keep games in their own Games folder. And then the dialogs are kind of annoying.
though preferences files were a bit of a mess.
I vaguely remember if early Macintosh System versions you would be prompted to insert the disk (with the correct disk name in the message) if you tried to open a file belonging to an application which was on an ejected disk.
Mac is now Prosumers and Professionals. And its UX should be treated as such.